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Monday, July 22, 2024

The Rapid Intensification of Hurricane Lee Is a Warning

Just a week after Hurricane Idalia “rapidly intensified” and slammed the Florida coast with monster storm surges, Tropical Storm Lee has grown into a massive hurricane in the Atlantic. By feeding on exceptionally warm waters, it has undergone rapid intensification, a transformation that scientists define as an increase in sustained wind speeds of 30 knots (35 miles per hour) or more within 24 hours. Lee boosted from 70 knots to 116 knots over just 12 hours yesterday. It’s now at 146 knots—a Category 5 hurricane—and is expected to intensify still more. In the Pacific, Jova rapidly intensified earlier this week from a 60-knot tropical storm to a 140-knot Category 5, which prompted one hurricane scientist to tweet: “Wait, what???” 

Such rapidly intensifying hurricanes are supposed to be exceptional. “Those are really rare,” says Jason Dunion, hurricane field program director at the National Oceanic and Atmospheric Administration’s Atlantic Oceanographic and Meteorological Laboratory. “If you remember growing up, the biggest kid in your class might be in the 90th percentile for height. The rapid intensifiers are in the 95th percentile of storms, as far as how quickly they intensify. They’re that rare. They really stand apart.”

At the moment, forecasters’ models are predicting that Hurricane Lee may pass north of the Leeward Islands, just east of Puerto Rico, but then curve north and miss the US East Coast. But there’s no guarantee: Hurricanes Irma in 2017 and Florence in 2018 were supposed to do the same but ended up ravaging Florida and the Carolinas, respectively.

Rapid intensification makes hurricanes extra dangerous because they change so quickly and dramatically as they approach the coastline. It’s a bit like watching a driver who’s cruising along at 25 miles per hour and then guns it right before hitting an obstruction. Residents might be expecting a storm they can ride out, but are instead faced with a full-scale hurricane that’s quickly grown monstrous.

“It’s that short period of intensification that can make it difficult to forecast, because there’s so many changes at once,” says Dunion. “That 35 miles per hour a day, that’s the equivalent of something like a Category 1 hurricane approaching landfall, and by the time it got there a day later, it’s actually a Category 3 major hurricane.” 

Forecasting is also difficult because rapid intensification is a complicated brew not just of atmospheric ingredients, but oceanic ones as well. To grow, a hurricane first needs warm water—in Lee’s case, the Atlantic. As water evaporates off the ocean surface, it rises as moist air, firing energy into the atmosphere. This creates a bubble of low pressure, which sucks in air, creating wind. More warm, moist air rises, condensing into thunderclouds that release heat. (Hurricanes suck up so much heat energy from the ocean, in fact, that satellite images show them leaving a trail of cooled water in their wake.)

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But a hurricane also needs humidity: If it runs into dry air, that can actually counteract warm waters, to a certain degree. “If it’s dry enough, you get really rapid cooling because of the evaporation, and you get downdrafts—that cooler air wants to sink,” says Dunion. “Downdrafts are just not what a hurricane wants to see if it wants to intensify. It’s all about the updraft.” 

Due to climate change, some parts of the world are indeed getting more humid as higher temperatures evaporate more water off the ocean surface. Generally speaking, a warmer atmosphere can also hold more water vapor than a cooler one: For every 1 degree Celsius of warming, you get 7 percent more moisture in the atmosphere. The oceans have also absorbed 90 percent of the additional heat that humanity has added to the atmosphere, providing all the more energy to supercharge hurricanes.

Today, Dunion’s team is flying out to Lee in a research aircraft that will parachute instruments into the storm to measure its humidity, along with wind speed, temperature, and pressure. They’ll also fly a drone closer to the ocean surface to measure how energy is being exchanged between the sea and the storm. “It’s really important to know while this storm is rapidly intensifying: How is that affecting the winds down at the surface? How quickly do the winds down at the surface respond to this rapid intensification?” asks Dunion. “That’s all important to the forecast.”

Another key variable in rapid intensification is land. Part of what has made Lee grow so strong is that it’s a “Cape Verde hurricane.” These form off the coast of Africa and head toward the Americas, feeding for thousands of miles on warm Atlantic waters—a constant source of energy, as these hurricanes don’t hit land until they cross the whole ocean. Once they do reach land, storms lose that source of energy. Mountains in particular can slice through such storms, weakening them. That’s why hurricanes lose strength as they travel through Southern states like Louisiana: Deprived of fuel, their winds peter out and they dump out their moisture as rain. 

Hurricanes also hate vertical wind shear—basically, differences in wind speed and direction at different altitudes. “If [winds are] too different, it’s almost like tipping a skateboarder over—the storm starts to get tilted and not really be able to strengthen,” Dunion says. Interestingly enough, early this summer, before hurricane season got going, scientists were speculating about whether El Niño might butt in and help break up this summer’s storms. That’s because El Niño—a band of warm water in the Pacific—tends to create wind shear in the Atlantic. But clearly, Lee doesn’t seem fazed. 

Putting it all together: To get rapid intensification, you need warm water, high humidity, and low wind shear. If you knock out any one of those variables, it’s a no-go. That’s what makes rapid intensification very rare. And even with all those variables lined up, rapid intensification isn’t a sure thing. “We don't have a deep understanding of the reason why we have this rapid intensification,” says University of Delaware atmospheric scientist Shuai Wang. “We can say: OK, now we have a high probability to have such events, but we are not sure whether or not it will happen.” 

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That’s why rapid intensification remains such a difficult thing to forecast. But scientists are using both decades-old data and new measurements to fix that. Last month, Wang published a paper in the journal Nature Communications analyzing the frequency of rapidly intensifying cyclones (“hurricane” is another word for cyclone) in the open ocean and within 250 miles of a coastline over the past four decades.

While Wang found no significant trend in the open ocean, the amount of rapid intensification exploded near coasts. (The graphs above show the number of rapid intensification events between the years 1980 and 2020. The bottom graph shows events where wind speed increased at least 30 knots in 24 hours, and the top shows even more extreme intensification of 45 knots.) 

Idalia was a prime example of this, rapidly intensifying as it neared the Florida coast. “Four decades ago, we had five rapid intensification events in the coastal offshore region annually. But now we have 15, so the number has tripled,” says Wang. “We think vertical wind shear weakening and humidity increases may be two important reasons why we are seeing this very significant trend in rapid intensification events.” 

Climate change, too, has been providing ever more heat energy for hurricanes to feed on: Earlier this summer, Florida logged water temperatures of 101 degrees Fahrenheit. Indeed, Wang’s analysis found that the rise in offshore rapid intensification could be due to both natural variability in the climate and human-caused climate change. While scientists will need to do specific studies to see how much climate change contributed to Idalia’s rapid intensification near the coast, it was a “scenario that we may see more in the future,” Wang says.

Similarly, climate scientist Karthik Balaguru, of the Pacific Northwest National Laboratory, has found that the Atlantic coast is becoming a breeding ground for rapidly intensifying hurricanes. Once again, the problem is expected to worsen with climate change. “We have identified warming of the sea, reducing wind shear, and also the atmosphere is becoming more and more moist,” says Balaguru. “All of these factors are becoming more and more favorable, making the environment in general more conducive for intensification.” 

The wind shear factor is particularly interesting because it begins on the other side of the country. Climate models predict that the eastern Pacific Ocean is going to heat significantly, with maximum warming just north of the equator. “It basically sets off waves in the atmosphere,” Balaguru says. “These waves, in turn, change the circulation in the upper troposphere above North America. And one of the consequences of these circulation changes is that the wind shear will likely reduce, especially near coastal regions.” On the Atlantic coast, this reduced wind shear would favor the rapid intensification of hurricanes nearing landfall. 

It’s yet another illustration of the confounding complexity of rapid intensification. But with more data, scientists can better understand the phenomenon and improve their models, giving coastal populations better warning of the monsters hurtling toward shore.

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